In recent years, electrically operated vehicles, such as electric vehicles, hybrid vehicles, and fuel-cell vehicles, have been developed and put to practical use, as environmentally friendly vehicles. The electrically operated vehicles generally include a motor for generating driving force or power, and a storage device that supplies electric power to the motor. In many cases, a secondary battery is used as the storage device of the electrically operated vehicle.
It has also been proposed to charge the storage device (battery) installed on the electrically operated vehicle, using a power supply located outside the vehicle. The electrically operated vehicle having this arrangement will be called “plug-in vehicle”. Also, charging the storage device installed on the vehicle by means of the external power supply will be called “plug-in charge”.
For the plug-in vehicle, it is desired to extend or increase the driving range or distance over which the vehicle is able to run using electric power stored in the storage device. To this end, two or more batteries, for example, are installed on the plug-in vehicle. For example, Japanese Patent Application Publication No. 2007-259612 (JP-A-2007-259612) discloses a control system for controlling a power supply having a plurality of batteries. The power-supply control system includes a detecting means for detecting the state of charge of each battery, and a charging/discharging means that determines a degradation condition of each battery, based on the output of the detecting means. The charging/discharging means controls a plurality of switches for connection between the respective batteries and a load, based on the degradation condition of each battery. The charging/discharging means controls the plurality of switches so as to equalize the degradation conditions of the respective batteries.
Japanese Patent Application Publication No. 2008-83022 (JP-A-2008-83022) discloses an evaluation system that evaluates a degradation condition of a battery installed on a plug-in vehicle. The evaluation system collects data while the battery is being charged by means of an external power supply located outside the vehicle, and evaluates a degradation condition of the battery, using the data.
For a plug-in vehicle having a plurality of secondary batteries, it is desired to appropriately control charge/discharge of the batteries, based on a degradation condition of each battery. For example, charge/discharge current of each battery, or the amount of electric power that can be supplied to or from the battery, is controlled according to the degradation condition of the battery. However, JP-A-2007-259612 discloses only the technique to select a battery to be used, from the plurality of batteries.
According to JP-A-2008-83022, a degradation condition of the secondary battery is evaluated during plug-in charge. During running of an electrically operated vehicle, frequent switching occurs between discharge and charge of the battery, and, furthermore, the magnitude of battery current varies frequently. It is, therefore, not easy to evaluate a degradation condition of the secondary battery during running of the electrically operated vehicle. On the other hand, the battery is in stable conditions during plug-in charge. Therefore, a degradation condition of the battery can be determined with high accuracy during plug-in charge.
However, when the interval between plug-in charges is long (for example, when there is little chance that the user puts the battery on charge), a deviation or difference between the degradation condition evaluated during plug-in charge and the current condition of the battery may be increased. In this case, too, it is difficult to appropriately control charge/discharge of the battery according to the current battery condition.